1. Field of the Invention
The present invention relates to a crystal resonator with plural electrodes for use in a crystal oscillating device. More particularly, the present invention pertains to a crystal resonator with plural electrodes for use in a crystal oscillating device of a temperature compensation type.
2. Description of the Related Art
A crystal oscillating device using a crystal resonator with plural electrodes has been developed by the present inventors, and is used specifically as a crystal oscillating device of a temperature compensation type. This crystal oscillating device is disclosed in, for example, U.S. patent application Ser. No. 07/382,107 (now U.S. Pat. No. 5,004,988).
FIG. 1 exemplifies this type of a crystal oscillating device. The crystal oscillating device comprises a crystal resonator 1 with plural electrodes, an oscillation circuit 2, a temperature compensation circuit 3, and a frequency adjusting element 4. The crystal resonator 1 includes a crystal piece 5 of, for example, an AT-cut type, which activates thickness shear vibration. Two pairs of electrodes, 6a and 6b, and 7a and 7b, are provided facing each other on both surfaces of the crystal piece 5 in the direction of the z' axis. The oscillation circuit 2 and the frequency adjusting element 4 are arranged in series between the electrode pair 6a and 6b. The temperature compensation circuit 3 is connected between the other electrode pair 7a and 7b. The oscillation circuit 2 oscillates a resonator by the electrodes 6a and 6b at a frequency which depends on temperature. The frequency adjusting element 4 is a variable capacitor which adjusts the oscillation frequency to a predetermined value. In the temperature compensation circuit 3, a compensation voltage oscillating circuit 8 generates a compensation voltage corresponding to the ambient temperature to change the capacitance of a voltage-variable capacitor 9.
In the crystal oscillating device with this structure, the change in the capacity of the voltage-variable capacitor 9 varies additional capacitance of the oscillation circuit 2 due to the elastic coupling of the two electrode pairs 6a and 6b, and 7a and 7b, thus compensating the frequency-temperature characteristic. Moreover, since the frequency adjusting element 4 is not electrically coupled to the temperature compensating circuit 3, the influence on the temperature compensation circuit 3 at the time of adjusting the frequency is reduced, thus enhancing the reliability.
Because the crystal resonator with the above structure uses the resonator 1 with plural electrodes, however, non-harmonic high-order vibration in an antisymmetric mode f.sub.a1 occurs near the main oscillation frequency f.sub.0 with respect to the main vibration (symmetric mode f.sub.s1) as shown in FIG. 2. A resonance characteristic shown in FIG. 7 is the result of the measurement conducted by a transmission characteristic method. This phenomenon will be explained as follows. The thickness shear vibration has countless modes represented by a mode symbol (y, x, z) where "y" is the order of an overtone in the direction of thickness, and "x" and "z" are the orders of the peak and valley of vibration in each direction. When the electrode pairs 6a and 6b, and 7a and 7b are arranged along the z' axis, two modes are generated: a symmetric mode f.sub.s in which "z" in the mode symbol is an odd number, and an antisymmetric mode f.sub.a in which "z" is an even number. Particularly, the symmetric mode f.sub.s1 (main vibration) with the mode symbol (y, 1, 1) as shown in FIG. 3A, and the antisymmetric mode f.sub.a1 (y, 1, 2), closest to the symmetric mode f.sub.s1, as shown in FIG. 3B are forcible generated. When there is only one pair of electrodes 10a and 10b as shown in FIG. 3C, the antisymmetric mode will not be generated because displacement-originated charges will cancel out each other.
Accordingly, the use of the above-structured crystal resonator causes a frequency pump that the oscillation frequency moves from the main vibration frequency f.sub.0 to the vibration frequency in antisymmetric mode f.sub.al.